Circular Switching Surface Technique: High-Performance Constant Power Load Stabilization for Electric Vehicle Systems

Electric vehicles make use of energy storage systems, such as batteries and/or ultracapacitors to power the electric power drive train, as well as auxiliary automotive system for control, safety, and comfort. This relatively complex power structure can be described as a distributed multiconverter system. The constant power behavior of tight-speed controllers in the vehicle´s traction system and tightly regulated dc-dc converters connected to the HV-DC bus produces instability effects. This paper proposes a simple and practical geometric control, using circular switching surfaces, to address constant power load instability in electric vehicle´s power systems. The proposed switching surfaces provide a solution in the geometrical domain to constant power loading conditions, while achieving outstanding dynamic response compared to state-of-the-art controllers. The controller is implemented in a bidirectional Buck + Boost cascade converter as a battery charge/discharge unit and ensures reliable system operation. The predictable and consistent behavior of the converter with constant power load is presented by analyzing the system curves in the normalized state plane with the switching surfaces employed. Simulation and experimental results on a scaled 1-kW Buck + Boost cascade converter validate the proposed switching surfaces and predictions regarding the converter´s behavior under constant power loading conditions.